Ground effect machine having heave stability for traversing rough surfaces



May 4, 1965 c. s. COCKERELL 3,181,636

GROUND EFFECT MACHINE HAVING HEAVE STABILITY Filed Sept; 29, 1960 FORTRAVERSING ROUGH SURFACES 6 Sheets-Sheet 1 DIRECTION OF TRAVEL OFVEH/CLE Inveof'ar C. S. COCKEREL A lfarnzys May 4, 1965 FOR Filed Sept.29. 1960 GROUND EFFECT q. s. COCKERELL 3,181,636

'IMACHINE HAVING HEAVE STABILITY TRAVERSING' ROUGH SURFACES 6Sheets-Sheet 2 Invenf'or C. S. COCKCRELL M) May 4, 1965 c. s. COCKERELL3,181,636

GROUND EFFECT MACHINE HAVING HEAVE STABILITY FOR TRAVERSING ROUGHSURFACES 6 Sheets-Sheet 5 Filed Sept. 29. 1960 FIG.6.

Inven C. S. Cocmsna 2a N m LL. WWW

flf/arneys May 4, 1965 FOR TRAVERSING ROUGH SURFA Filed Sept. 29. 1960 6Sheets-Sheet 4 FIG.I4.

In VcnTo C, S. C oCKERELL.

c. -s. COCKERELL 3,181,636 i GROUND EFFECT MACHINE HAVING HEAV TABILITY5 M, 42. plaza May 4, 1965 c. s. COCKERELL 3,181,636

GROUND EFFECT MACHINE HAVING HEAVE STABILITY FOR TRAVERSING ROUGHSURFACES Filed Sept. 29, 1960 6 Sheets-Sheet 5 FIGJS.

. 92 93 L Y/ M I [Fr/1 /////1 l\ \\Y/// 43\ \3] ,1 Q

94 FIG.I7A.

Inve 'far C. S. COCKL-ZRELL.

y 4, 1965 c. s. COCKERELL 3,181,636

GROUND EFFECT MACHINE HAVING HEAVE STABILITY FOR TRAVERSING ROUGHSURFACES Filed Sept. 29, 1960 6 Sheets-Sheet 6 FIG.I9.

Invenf'or C. S. COCKERELL.

United States Patent 3,181,636 GROUND EFFECT MACHINE HAVEJG HEAVESTABILITY FOR TRAVERSING ROUGH SURFACES Christopher Sydney Coclrerell,East Cowes, Isle of Wight, England, assignor to Hovercraft DevelopmentLimited, London, England, a British company Filed Sept. 29, I960, Ser.No. 59,306 Claims priority, application Great Britain, Oct. 2, 1959,33,535/59 9 Claims. (Cl. 180-7) This invention relates to vehicles fortravelling over land and/or water and which are supported above thesurface of the land or water by one or more cushions of pressurizedfluid.

In such vehicles means are provided for forming and maintaining acushion or cushions of pressurised fluid beneath the vehicle, such meansgenerally being adjacent to the periphery of the bottom of the vehicle.

The above described method of support is also applicable to a platformprimarily intended to remain stationary, for example, for supporting aradar installation, and the term vehicle as used herein is to beunderstood as including, where the context permits, a platform.

The invention is concerned with the heave stability of the vehicle whentravelling over rough water or over ground contoured in a similarmanner. When the vehicle is travelling over rough water or ground, thepath of the centre of gravity of the vehicle is determined by certainparameters, some of which are design parameters and others depend uponthe speed of the craft, the length of the craft, and the height andspacing of the obstacles forming the rough surface. Dependent upon theseparameters, such a path may be acceptable, but often the combinations ofthese parameters may result in an unacceptable path, due to such pathexhibiting excessive heave, for example, a resonant condition or a pathwhich is too far out of phase with the contours beneath the craft. Theseundesirable eifects may be reduced by an appropriate dynamicmanipulation of some of the design parameters which determine the totalupward thrust of the pressurised cushions at any one instant of time,thus enabling the total upward thrust to be varied.

It is well known that a rough surface, whether it be land or sea can bereproduced by the summation in appropriate phase and amplitude of aseries of component waves superimposed upon one another. Thus although aWave formation is in general the result of the superimposition of anumber of individual wave formations of difierent length and height andspeed of translation, it is permissible, in order to simplify theanalysis, to consider simple formations first of short and then of longwaves, it being known that a control system which is suitable for eitherseparately will give a sat-isiiaotory performance when more than one ispresent, provided the amplitudes of the components in combination arewithin the maximum for which the control is designed.

The problem can be illustrated by considering a number of varyingoperating conditions. Taking first when the vehicle is traversing plainwaves, whose Wavelength is short compared with the length of the craft,then the integrated lift of its supporting cushion will remain verynearly constant, and so the path of the centre of gravity of the vehiclewill be very nearly level, and the form of this path will be very littlealfected by the speed of the vehicle. However, when the vehicle isslowly traversing plain Waves whose length is very long compared withthe length of the vehicle, the vehicle will heave in phase with the waveso that the hover-height will be very nearly constant with respect tothe mean height of the section of the wave beneath the vehicle in anyone instant. Again,

3,18l ,636 Fate-rated May 4, 1965 when the vehicle is quickly traversingplain waves whose length is very long compared to the length of thevehicle, the vehicle will heave but the phase of its path will lagbehind that of the wave so that its mean hover-height halfway down thewave will be greater than its mean hover-height when climbing the nextcrest. When the vehicle is traversing plain waves of intermediate lengththe vehicle will heave, in general, by less than the height of thewaves, both the amplitude and the phase of this heaving path dependinggreatly upon the speed of traverse, the length of the craft and thenatural period or periods in heave of the craft and its air cushion. Ina further condition, when the vehicle is traversing plain waves ofintermediate length and the frequency of encountering them is of theorder of the natural frequency of the vehicle in heave, or a multiple ofit, the vehicle is excited by the waves and a resonant condition isobtained, resulting in an amplitude of heave which may be far greaterthan the wave height.

In general, the fluid forming the cushion or cushions is air, andsimilarly, where fluid curtains are used to contain the cushion orcushions, these are also formed by air, and for convenience hereinafterthis will be considered to be the case. However, other fluids, such asexhaust gases and Water or steam can be used particularly for formingthe curtains.

The invention will be readily understood from the following descriptionof various embodiments in conjunction with the accompanying drawings, inwhich:

FIGURE 1 is a diagrammatic vertical cross-section through a typicalsurface undulation showing the path followed by the centre of gravity ofthe vehicle without the application of the invention,

FIGURE 2 is a vertical cross-section of a vehicle embodying one form ofthe invention,

FIGURE 3 is a vertical cross-section of a slightly different form ofvehicle embodying a similar form of invention as shown in FIGURE 2,

FIGURE 4 illustrates a modified form of the vehicle illustrated inFIGURE 2,

FIGURE 5 is a similar cross-section as in FIGURE 2 illustrating analternative form of the invention,

FIGURE 6 is a similar cross-section as in FIGURE 3 illustrating theapplication of an alternative form of the invention as shown in FIGURE3,

FIGURE 7 is a plan view of the vehicle as shown in FIGURE 2,

FIGURE 8 is a diagrammatic view of an apparatus for positivelycontrolling the operations of the valves shown in FIGURES 2 to 6,

FIGURE 9 is a vertical cross-section of a vehicle similar to that shownin FIGURE 5, having an alternative fluid curtain system,

FIGURE 10 is a vertical cross-section of a vehicle sim ilar to thatshown in FIGURE 4 illustrating a modification thereof,

FIGURE 11 is a diagrammatic partial cross-section of the bottom part ofthe vehicle embodying a further form of the invention,

FIGURE 12 is a modification of FIGURE 11,

FIGURE 13 illustrates a further modification of FIG- URE 11,

FIGURE 14 shows a further form of embodiment shown in FIGURE 11,

FIGURE 15 is a partial plan view of the detail of a flap as used in thevarious forms of the embodiment shown in FIGURES 11 to 14,

FIGURE 16 is a diagrammatic partial cross-section of the bottom of thevehicle having yet another embodiment of the invention,

FIGURES 17A and 17B illustrate diagrammatically a further embodiment ofthe invention,

periphery of a vehicle illustrating a further embodiment of theinvention, and

FIGURE l9illustrates a modification of the. embodiment illustrated inFIGURE 18.

As stated above when a vehicle is traveling over an undulating surface,or when a vehicle is hovering over water with waves flowing beneath it,the effect of the variation in height, or clearance, between the bottomof the vehicle and the surfacewill depend upon the wave length of theundulations compared with the length of the vehicle. Where the wavelength is a sub-multiple of the vehicle length, the vehicle willmaintain a substantially level course, rising and falling onlywhenencountering isolated taller crests or deeper troughs ofsuchundulations. At the other extreme, where the wave length is severaltimes the length of the vehicle, the vehicle will follow the profile ofthe undulations without undue vertical accelerations cushion, the airbeing supplied to the port 3 from a duct 5. In FIGURE 3 the cushion iscontained peripherally by a flexible downward projecting peripheralmember 6, air being blown into the space occupied by the cushion througha port 7 from a duct 8 in the bottom of the main body of the vehicle,existing air escaping beneath 'the downward projecting member'6. Inbothvehicles, one way valves 9 and 10 are provided which in their normalposition close ports or ducts 11 through the downward projecting members'4 and 6. The valves 9, at the front of the vehicles, open inwardstowards the cushion, while the valves 10, at the rear of the vehicles,open outwards. The valves 9 and 10'ma'y, of course, be loaded or biasedtoa ward their normally closed positions in any suitable manbeingimposed on the vehicle. I have found that the range of wave lengths atwhich unacceptable vertical accelerations may occur generally lies veryroughly between 1 and 2 /2 times the length of the vehicle, and ifprovision can be made to cause the vehicle to follow a modified pathhaving a reduced height variation for this range of wave lengths, theundue vertical accelerations which would otherwise occur can be reducedto an acceptable value, or

even almost prevented. a FIGURE 1 illustrates diagrammatically. theprofile of an undulating surface, the wavelength being of the order oftwice the vehicle length, together with an indication of the path whichwould normally be followed by the centre of gravity of the vehicle, Asdescribed hereinafter, the undulating surface will be considered asbeing waves in water although it willbe appreciated that the inventionis applicable to a vehicle travelling over other surfaces hav-' ingsimilar characteristics such as sand dunes and the like.

Line A represents the profile of a wave andthe chain dotted line B isthe path followed by the centre of'gravity of a vehicle which wouldnormally occur. At the'positions indicated at C, D and E the height isthat which it is, intended to operate the vehicle, that is the cushionvolume. and mean pressure and the curtainheight, areat their correct ordatum values. It will be seen that as the centre,

of gravity of the vehicle passes pointC the height of the vehiclerelative to the surface increases beyond the mean. and there is acorresponding increase in cushion volume which causes a decrease incushion pressure below the mean. This increase in volume andthe;accompanying decrease in cushion pressure results in a decreased upwardthrust from the vehicle which continues until the centre. of gravity ofthe vehicle reaches point D.- As the centre of gravity passes point D,the height of the vehicle decreases below the mean, with a correspondingdecrease in cushion volume and an increase in pressure beyond the mean,and an increase in upward thrust, until pointE is reached. Thevariations in upward thrust causeyariations of the lift exerted from thevehicle which cause, vertical accelerations which may be quite large.

To reduce or prevent these variations inlift with the resultantvariations in upward thrust, the variations in cushion pressure arereduced or prevented. Thus, for ex.-v

ample, by' providing additional air to the cushion for the period thecentre of gravity of the vehicle is travelling from C to D, and bytheextracting air from the cushion for the period the vehicle istravelling from D to E, its

to hereinafter as such. The air curtain 2 flows down-1 ner, as bysprings (not shown). If the pressure of the cushions 1 falls belowtheatmospheric, theffront valves 9 open and admit air to the cushion,reducing the loss ,of lift and the normal result of the downwarddropofthe vehicle. Similarly, when the pressure of the cushions 1 exceeds thepredetermined value, rear valves 10 open to allow the escape of some ofthecushion forming air to atmosphere. It is advisable to provide aseries of ports or ducts 11 as shown in FIGURE'7 described below toenable large flows of air to' take place.

As indicated above, the effect on the vehicle of variations in thepressure of the cushion and in resultant thrust on the vehicle bottom isexactly the same when the vehicle is stationary and hovering over waterwith waves flowing therebeneath as when the vehicle is travelling overan undulating surface, whether thatsurface be stationary or moving. 7 a

Assuming that the vehicle is hovering, when the crest of a wave passes'therebeneath and causes the vehicle to move'upwards, there is anovershootingeffect due to the inertia of the vehicle whichproduces adecrease in cushion pressure. With a sufficiently fast movementandsteepness ofthe wave, the cushion pressure can fall below atmosphericpressure, whereupon the vehicle becomes subject to a downward forcewhich accelerates the vehicle downwards; As the vehicle moves downwards,the volume of the cushion starts to decrease and the cushion pressurewill'start to increase. When the vehicle has dropped to a height abovethe surface of the wave equivalent to the normal hoverheighh'the cushionpressure will be' normal and the vehicle will have reached its maximumdownward velocity. As the vehicle continues to drop, the clearance andthe cushion volume will decrease with a'corresponding increase incushion pressure, which increase in pressure slows down the verticalmovement of the vehicle until it is finally arrested. Since the increasein pressure is proportional to the decreasein clearance, the rise incushion pressure is relatively rapid. Forexam'ple, assumingv that thenormal'clearance or hoverheight of the vehicle is 12', an initialdecrease in clearance of 6" doubles the cushion pressure, a furtherdecrease of only 3" doubles the pressure again, whilea still furtherdoubling of pressure occurs for only another 1 /2"v decrease inclearance. It

. should therefore be evident that, even though the cushion pressure maydropbelow atmospheric pressure, the vehicle itself will not drop downonto the surface of the,

water, but its downward movement will be arrested by the very muchincreased cushion pressureproduce'd during the Idroppingmovement, whichpressure will then accelerate the vehicle upwards.

As pointed out below,'the only material difference between conditionswhen the vehicle is travelling over an undulating surface and when it ishovering over water with waves flowing beneath it, is that whenithevehicle is wardly from an annular supply port 3 formed in the bot-i :tomof 'the' downward projecting peripheral member 4 which also peripherallycontains .thelupper portion of the moving it-is possible to takeadvantage of;the. head pressure produced at the front of the vehicle wasto feed'air to the cushion before its pressure drops below atmosphericpressure. There may also be aregion of subat'rnospheric pressure at therear -of the vehicle whichcan be utilized'to facilitate the flow ofairout of the cushion when its pressure increases above a predeterminedvalue.

The objective of the present invention is thus to reduce the verticalaccelerations imposed on the vehicle by reducing the variations incushion pressure. This is accomplished by admitting air to the cushionwhen the pressure rops below atmospheric, and by releasing air from thecushion when the pressure goes above some predetermined value, wherebythe vertical movements of the vehicle are damped or leveled out.

Such a system as shown in FIGURES 2 and 3 reduces to some extent thepressure variations and the path of the centre of gravity of the vehicleis slightly modified from that shown in FIGURE 1, but variations incushion pressure and thus lift will still occur. This is partly due tothe fact that no air is admitted to the cushion until it is belowatmospheric pressure. A further improvement can be obtained if it isarranged to admit air to the space occupied by the cushion 1 before thepressure drops below atmospheric. A source of high pressure is thusrequired. Still further improvement is possibl'= if it is arranged toexhaust air from the cushions to a region of reduced pressure. In thevehicles as shown in FIGURES 2 and 3, these improvements can readily beachieved when the vehicles are in motion as the pressure at the front ofthe vehicle is above atmospheric, whilst the pressure at the rear of thevehicle is slightly sub-atmospheric.

The vehicles shown in FIGURES 2 and 3 have the dis advantage that asubstantially rigid downward projecting member, comprising the whole ofthe member 4 in FIG- URE 2 and a comparatively large part of the member6 in FIGURE 3, must be provided, increasing the height of the structure.

To avoid this, ducts or ports 12 can be formed in the main body of thevehicle as shown in FIGURE 4. In this example, ducts 12 are formed inthe periphery of the vehicle, valves 13 being provided in the ductsacting in a similar manner to the valves 9 and of FIGURES 2 and 3.Although FIGURE 4 illustrates a modification of FIGURE 2 a similarmodification can also be made to FIGURE 3.

An alternative arrangement is to provide ducts through the body of thevehicle, the ducts serving both for the influx and efiiux of air,controlled by a valve capable of opening either way. Such an arrangementis illustrated in FIGURES 5 and 6, the vehicle shown in FIGURE 5 beingsimilar to that shown in FIGURE 2, whilst the vehicle shown in FIGURE 6is similar to that shown in FIG- URE 3.

In the vehicle shown in FIGURES '5 and 6 the duct 15 is formed in themain body 16 of the vehicle, communicatiilg at one end with the spaceoccupied by the cushion 1, and at the other end with the surroundingatmosphere. Two valves 17 normally close the ducts 15 under the actionof springs 18 and can rotate one way or the other, as indicated by thedotted lines 19 to allow air to flow into or out of the cushion space.Valves 17 can be so controlled by springs 8 that there is restrictedpressure range over which they remain closed. It will be seen that inFIG- URE 5 the supply port 20 need only project slightly below thebottom of the vehicle or may be flush with the bottom surface.Similarly, in FIGURE 6 the downward projecting member 21 need not be aslong as member 6 in FIG- URE 3. Instead of one large duct 15 beingprovided a number of smaller ducts can be used.

FIGURE 7 illustrates diagrammatically the relative position of the ducts11 to the vehicle illustrated in FIG- URE 2. Ducts 11 are positioned atthe front and the rear of the vehicle as shown. The air for theformation of the air curtain 2 enters through intakes 22, being forcedby the propellers 23 into the duct 5. A plan view of the vehicle shownin FIGURE 3 will be very similar, the difference being that the air fromthe propellers 23 is fed to duct 8 instead. Again the plan form of thevehicle as illustrated in FIGURES 5 and 6 is similar to that of FIG- URE7.

The valves 9, III, 13 and 17 of FIGURES 27, instead of being springbiased or controlled, may be positively actuated by a device or devicescontrolled, for example, by cushion pressure. Such a device is showndiagrammatically in FIGURE 8. An hydraulic control valve illustratedgenerally at 39, controls the flow of pressurised oil from supply inlet31 to either side of the piston 32 mounted in a cylinder 33. Piston 32is connected by a suitable linkage 34 to a valve 35 which corresponds tothe valve 17 of FIGURES 5 and 6. The cushion pressure I" is fed to aninlet 36 of the control valve 30. Springs 37 on either side of thepiston tend to maintain the piston in central position, in which thevalve 35 is closed.

In operation, variations in cushion pressures are fed by inlet 36 to thecontrol valve 30, oil being supplied to one side or the other of piston32. The piston moves in the cylinder, rotating a valve 35 one way or theother. On return of the cushion pressure to normal the oil supply tocylinder 33 is cut off and both sides of the piston open to exhaust,whereupon the springs 37 return the piston to its central positionclosing the valve 35.

Where the valve 35 needs to move only in one direction, as for examplein FIGURES 2, 3 and 4, only a single-sided piston 32 is required.

FIGURE 9 illustrates the application of the invention to a vehiclehaving a more complex form of air curtain, such as described in patentspecification of co-pending application Serial No. 837,428, filedSeptember 1, 1959, and since abandoned. In such curtain systems, atleast part of the curtain forming air is recovered through recoveryports formed at the bottom of the vehicle, inboard of a supply port.This vehicle is similar to the vehicle illustrated in FIGURE 5, the airrecovered through the recovery ports 40 being re-energised andrecirculated back to the supply port 3.

In a vehicle having an air curtain system, such as in FIGURE 2, 4, 5 and9, it is possible to reduce the mass flow of the curtain at anyconvenient position, e.g. at the rear of the vehicle, thus weakening it,when the pressure of the cushion tends to rise above the predeterminedpressure. Air from the cushion can then escape beneath the curtain. Thisis illustrated diagrammatically in FIGURE 10. A mass flow of air throughthe supply port 43 can be varied locally by a sliding flap 44. In theexample shown, fiap 4-4 at the rear of the vehicle, on the right ofFIGURE 10 is restricting the How of air to the part of the supply port43 at the rear, locally weakening the curtain of air 45. Air from thecushion is able to flow out beneath the air curtain as at 46. One ormore additional air curtains may be formed beneath the vehicle tosubdivide the cushion, as shown dotted at 48.

It will be apparent from the foregoing description of variousembodiments in the invention that the invention seeks to reduce, preventor even reverse variations in vertical thrust due to the variations incushion pressure, which would otherwise occur, when traveling over wavesand obstacles of similar profile. Thus, when the vehicle leaves a crestof the obstruction and travels over a trough,

the cushion pressure is maintained, as by the valves 9.

in FIGURES 2 and 3, at a pressure above that which would normally occur,up to the mean pressure, or may be even increased above the mean.However, the curtain height is greater than normal at this time and maynot be capable of containing the required cushion pressure. Conversely,when the vehicle after traveling over a trough encounters a rising faceof an obstacle, the cushion pressure is maintained, as by the valves 10in FIG- URES 2 and 3, at a pressure below that which would normallyoccur, down to mean pressure, or may be even ecreased below the mean. Atthis time, however, the curtain height is lower than normal and iscapable of maintaining and will tend to maintain a cushion of pressureabove the mean.

, The application of the variation of themass flow of the curtainforming air is similar to the variation of mass flow asdescribed above,with reference to FIGURE 10. Servo motors 47, are provided to operatethe flaps 44, 'in combination with the above described variation of massflow for the admission of air to, and exhausting of air from, thecushion again controlled by variations in cushion pressure.

As stated above the necessity for the control according to the inventiondecreases as'the length of the wave increases, in that'with very longwaves undue accelerations will not be generated if the vehicle isallowed to follow their contour.

-The variation of vertical thrust, or lift, produced by the cushion canalso be obtained by varyingthe area of the cushion. For example, ifthecushion pressure has decreased and it is desired to reduce, orprevent,,the loss of vertical thrust which would otherwise occur, thiscan be done by increasing the effective area of the cushion,

such as by moving the air curtains outwards or by providing a furthercurtain out-board of the existing curtain. The converse applies when itis desired to prevent or reduce the increase in vertical thrust whichwould otherwise occur on an increased cushion pressure. Various ways ofvarying the efiective cushion area are described below. a f a FIGURE 11shows a simple arrangement in which two supply ports 54 and 55 aresupplied with air by ducts 56 and 57 respectively. A hinged flap 58 ismounted at the junction-of the two ducts 56 and 57,:the position of theflap controlling the amount of air fed from a compressor by a duct 59,which enters each of ducts 56 and. 57. By rotating the flap'one way,anti-clockwise in FIG- URE 11, the amountof air flowing tothe inner-aircur- .tain'is reduced and that flowing to the outer air curtain is.increased. This leads to an increase in pressure of the secondarycushion which is' formed between the 'two air curtains and thusincreases the effective area of the main cushion. Reverse rotation ofthe flap has the reverse elfectl FIGURES 12 .and 13 are variations ofthe shown in FIGURE 11. In these examples the outer air curtain isformed from two supply ports 60 and 61, fed by two ducts 62 and-63respectively, the inner. air curtain being formed from a single supplyport' 64 fed by a duct 65. A hinged flap 66 operates to vary'the airfiow to'the two ducts 63 and 65, the air flow tothe duct 62 beingseparate. Operation of the flap varies the relative air system gPorts 88and 89 are postioned in the plate 85 so that when.

flow to the two supply ports 61 and 64 with the above described efie'ct.In FIGURE12 the ducts 62 and 63 are raised so that air from thecompressor flowing along the duct flows smoothly into the duct 65. InFIGURE 13 the ducts'62 and 63 are not raised and a curved guide isformed from two supply ports, the outer air curtain by supply ports 70and 71 fed by ducts 72 and .73 and the inner air curtain by supply ports74 and 75 fed by ducts 76 and 77 respectively; In each pair of ducts theouter duct 72 and 76 is wider than the inner duct. A hinged 8 her, onlythe relative flows to the outer ducts 72 and '76 being varied. V I a Inthe arrangements as shown in FIGURES 11 to 14, where the flap or valveis circular it must be made of flexible material or sectors of stifimaterial with radial joints of flexiblematerial as shown in FIGURE 15,which is a plan view'bf a suitable flap for use in FIGURE 13. FIGURE 16illustrates diagrammatically an alternative method of varying relativeflows of air to the supply ports, using a sliding flap or plate. A flatplate is mounted on the bottom surface of the vehicle, arranged to slideinwardly and outwardly over-the supply ports 86 and 87. The supplyports86 and 87 are much wider in the radial direction than is normally thecase, i.e., in comparison with the supply ports of FIGURES 11-14, andthe sliding I plate :85 is provided with ports 88 and 89 which act asthe actual ports through which the curtain forming air issues. The ports88 and 89 are slightly wider than normal ports, being sufiiciently widerfor the issue of the maximum additional air it is intended should betransferred.

the plate is in its central position the outer edge of the port 88 isslightly outside the outer edge of the port 86 and the inner edgeof port.89 is slightlysinside the inner edge of port 87, the unobstructedwidths of the ports 88 and 89 being the correct widths for the formationof normal curtains. Air is fed to the ports via a duct 90 and it will beseen that the movement of the plate 85 in or out will vary the relativeflows of air out of ports 88 and 89. Vanes may be provided in the wideports 86 and 87. Whilst the flap plate is shown as being fitted insidethe bottom of the vehicle, it can readily 'be fitted on the outside, butis liable to be damaged. Again, the relative positioning's of the wideand narrow ports can be reversed, the narrow ports 88 and 89 beingformed in the bottom of the vehicle and the wide ports 86 and 87 beingformed in the sliding plate 85.

It will be understood that air is deflected from the inner air curtainsystem to the outer air curtain system when it is desired to increasethe effective area of the cushion to offset any decrease, in cushionpressure. The vehicle would normally be arranged tooperate with thepressure of the secondary'cushion formed between the air curtainsys'tems at, for example, half the pressure of the main cushion. Itwould then be'possible to increase the vertical thrust above the normal,or to increase the vertical thrust that would otherwise obtain, byraising the pressure'of the.

' 17A and 17B illustrate diagrammatically such a method for movingtheposition of a port relative to the bottom of the vehicle; The normalnarrow port through which the air curtain is formed is replaced by awide port 91. Mounted over the port is a slidable flap plate 92 having anarrow port 93. Port 93 is equivalent to the normal port 1 formed in thebottom of the vehicle'in the previously described vehicles, and wherethe air issuing through it forms a primary air curtain, port 93 is inthe form of an annulus or is aseries of ports in annular configuration.As shown valve member 78, formed from two parallel flaps, 78a

and 78b, is'mounted at the junction of the ducts, movement of the valvemember varying the flow of air into, the

outer duct of each air curtain forming system. Air passesbetween theparallel flaps 7 8a and 7 812 into the inner duct 73 of the outer aircurtain system andflows below the valve into the inner duct 77 of theinner air curtain sysin FIGURE 17A, the port 93 is-in its centralposition. By moving the plate in or out, in a general radial direction,the position of the portand thus the curtain can bevarled. Ifthe port 93is moved inwards towards the centre of the vehicle as shown in FIGURE17B, the edge of the cushion will be moved inwards which will have. theeffect of reducing the area of the cushion. Conversely moving the port93 outwards away from the centre of the vehicle has the effect ofincreasing the area of the cushion. The movable plate 92 is easilyprovided for straight portions of the ports at the bottom of thevehicle; where the. Ports are tem. The air flow into each of the innerducts73 and 77 I is virtually unafiected by any movement of the valvemem-,

curved, it will be necessary to provide for the plates to slide one overthe other as they are mov in andput to allow for the variation incircumferential distance. Vanes 94 may be provided in the wide port 91.

An alternative method of varying the effective area of the cushion, asapplied to curtain systems in which at least part of the curtain formingair is recovered, is illustrated in FIGURE 18. A supply port 100 isformed in the bottom of the vehicle at the periphery and a recovery pont101 is also formed in the bottom of the vehicle slightly inboard of thesupply port 100. A further recovery port 102 is formed in the bottom ofthe vehicle spaced further inboard than the first recovery port 101. Therecovery ports 101 and 102 communicate via ducts 103 and 104respectively with a common recovery duct 105. A sliding vane 106 ismounted between the two recovery ports and acts to vary the opening ofthe ports. In operation, air issues from the supply port 100, and someof the curtain forming air is recovered through recovery port 101,further air being recovered through recovery port 102. The sliding vane106 can completely close either of the recovery ports, and if, forexample, the outer recovery port 101 is closed then the cushion extends,effectively, only as far as the inner recovery port 102. Similarly, ifthe inner recovery port 102 is closed, the cushion extends as far as theouter recovery port 101. A graduation of the effective cushion areabetween these two limits can be obtained by varying the sliding vane106.

FIGURE 19 illustrates a method of cushion area variation similar to thatshown in FIGURE 18, a rotatable vane 107 being provided to vary therelative amounts of air recovered through the recovery ports 101 and102, instead of sliding vane 106.

As described above, when the vehicle is quickly traversing plain waveswhose length is very long relative to the length of the vehicle, thevehicle is liable to become out of phase with the waves, overshooting atthe crests and for at least part of the downward going faces, andundershooting at the troughs and for at least part of the upward goingfaces of the waves. Under such conditions it is possible that thevehicle will meet obstacles such as small waves imposed on the longWaves, which it would normally miss, but which due to the decreasedheight at certain positions in its path it will hit. This can beprevented if means are provided, for example, for increasing the massflow of the curtain forming air or increasing the cushion area so thatthe vehicle will tend to be maintained at its correct height. Theconverse will apply on the downward going slope of the wave and the massflow of the curtain air or the cushion area can be reduced at thesetimes.

I claim:

1. A vehicle for travelling over a surface comprising a body, means forforming and maintaining a cushion of pressurised fluid beneath said bodyby which said vehicle is at least partly supported above said surface asit travels thereover, and means carried by said body for reducingunacceptable vertical accelerations of said body, said means includingmeans responsive directly to variations in the volume of said cushionfrom a predetermined value for automatically supplying fluid to andreleasing fluid from said cushion and thereby reducing the variations invertical thrust on the vehicle produced by variations in the cushionpressure resulting from said variations in volume of the cushion.

2. A vehicle for travelling over a surface comprising a body, teams forforming a cushion of pressurised fluid beneath said body by which saidvehicle is at least partly supported above said surface as it travelsthereover, means for containing said cushion including a portion of thevehicle body, and means carried by said body for reducing unacceptablevertical accelerations of said body, said 'rneans including meansresponsive directly to variations in the pressure of said cushionrelative to the pressure of the atmosphere around the vehicle forautomatically supplying air from the atmosphere to said cushion andreleasing air from said cushion to the atmosphere and thereby reducingthe variations in vertical thrust on the vehicle produced by saidvariations in the cushion pressure.

3. A vehicle as claimed in claim 1 wherein said first mentioned meansincludes means for forming at least one curtain of fluid which at leastpartly forms and contains the cushion of pressurised fiuid.

4. A vehicle as claimed in claim 2 wherein said lastrnentioned meansincludes at least one opening in the body of the vehicle providingcommunication between said cushion and the atmosphere, and a flapmounted in said opening operative in response to variations in thecushion pressure for admitting air into the cushion when the cushionpressure decreases, and releasing air from the cushion when the cushionpressure increases, from a predetermined pressure.

5. A vehicle as claimed in claim 4 including power operating means foroperating the flap.

6. A vehicle as claimed in claim 2 wherein said lastmentioned meansincludes openings in the body of the vehicle at the front and rearthereof providing communication between said cushion and the atmosphere,a flap for admitting fluid into the cushion positioned in the opening atthe front of the vehicle, and a flap for releasing fluid from thecushion positioned in the opening at the rear of the vehicle.

7. A vehicle as claimed in claim 1 wherein said first mentioned meansincludes means for forming at least one curtain of fluid which at leastpartly forms and contains the cushion of pressurised fluid, and whereinsaid lastmentioned means includes means responsive to variations in thecushion pressure for varying at least locally the mass flow of the fluidforming the curtain, said last-mentioned means being so constructed andarranged as to decrease the mass flow when the cushion pressure tends toincrease and to increase the mass flow when the cushion pressure tendsto decrease, whereby fluctuations in cushion pressure are reduced.

8. A vehicle as claimed in claim 7 wherein said curtrain-forming meansfurther includes at least one supply port formed in the bottom of thevehicle, and wherein the means for varying the mass flow of fluidforming the curtain includes at least one movable member for varying thewidth of the supply port.

9. A vehicle as claimed in claim 8 wherein the means for varying themass flow of fluid forming the curtain further includes power operatingmeans responsive to variations in the cushion pressure for moving saidmovable member.

References Cited by the Examiner UNITED STATES PATENTS 1,123,589 l/15Porter. 2,567,392 9/51 Naught. 2,751,038 6/56 Acheson. 2,838,257 6/58Wibault. 3,042,129 7/62 Wade 7 FOREIGN PATENTS 219,133 11/58 Australia.1,238,499 7/60 France.

OTHER REFERENCES Popular Science, July 1959, pages 51, 52, 53, 54, 55,194.

A. HARRY LEVY, Primary Examiner. PHILIP ARNOLD, Examiner.

1. A VEHICLE FOR TRAVELLING OVER A SURFACE COMPRISING A BODY, MEANS FORFORMING AND MAINTAINING A CUSHION OF PRESSURIXED FLUID BENEATH SAID BODYBY WHICH SAID VEHICLE IS AT LEAST PARTLY SUPPORTED ABOVE SAID SURFACE ASIT TRAVELS THEREOVER, AND MEANS CARRIED BY SAID BODY FOR REDUCINGUNACCEPTABLE VERTICAL ACCELERATIONS OF SAID BODY, SAID MEANS INCLUDINGMEANS RESPONSIVE DIRECTLY TO VARIATIONS IN THE VOLUME OF SAID CUSHIONFROM A PREDETERMINED VALUE FOR AUTOMATICALLY SUPPLYING FLUID TO ANDRELEASING FLUID FROM SAID CUSHION AND THEREBY REDUCING THE VARIATIONS INVERTICAL THRUST ON THE VEHICLE PRODUCED BY VARIATIONS IN THE CUSHIONPRESSURE RSULTING FROM SAID VARIATIONS IN VOLUME OF THE CUSHION.